923A1 Electrical mystery and no-start?

[DampLemonade]

Hey all

My truck wouldn't start the other day. The starter motor made no clicking sound or anything which I thought was strange. I then noticed that turning the battery lever off does NOT turn off power to the truck, as in, the gauges stay powered, the buzzer still buzzes with the ignition in the "run" position, etc. I can only get electricity to shut off by removing the battery negative. Additionally, even with having the main negative cable removed, but with the NATO Slave plug still attached, i can still turn on power to the truck via the lever, and again can only turn off battery power by removing the NATO Slave cable. This makes no sense as there's nothing daisy-chained off the NATO port, but here we are anyway.

My initial reaction to the no-start would be to look for a new starter relay, but the whole battery negative mystery is really concerning me. Where would I even begin to look for the cause of this? The truck ran completely normal up until now.

 

[ericp351]

Look to the basics- You know that you have something wrong at your Dash Disconnect Switch, right? If so- check power in at the switch - if OK, check out. Remember- even a part connection/ short will pass the itty bitty current needed to light your dash. It should not be too hard to "jump out" any one switch or connection- just remember that a dropped Hot wire to your grounded truck metal is a big problem- 4 batteries hold lots of energy. You can start at the battery connections and go connection by connection to pin point trouble before you need to buy any parts. Rember the grounds! Will do no harm to run a heavy jumper from your main battery "earth" to your engine block. If very cautious, just "tap" it to block and look for spark check before you clamp. Lastly, for step 1- sometimes dirt can stop even a beefy bolt to properly pass a solid ground to your starter- unlikely with a 5 ton, but can happen. Best luck- can be a small, cheap trouble!

 

[Jbulach]

As for disconnecting your batteries at the negative, you must disconnect your slave connector cable. The slave connector is grounded to the truck body at the connector itself.

 

[fuzzytoaster]

You PCB is failing and causing your issue. Give it some love taps with a hammer to see if it turns off with the levers in the off position. Replace at when able . Also, Jbulach is correct with his ground diagnostics. It's working as intended. Ground is a ground in this regard. Remove both.

 

[DampLemonade]

The slave connector is grounded to the truck body at the connector itself.

IDK why I didn't think of this before but that makes so much more sense.

You PCB is failing and causing your issue. Give it some love taps with a hammer to see if it turns off with the levers in the off position. Replace at when able.

I poked around the TMs and with my multimeter and looks like you're right. Thankfully it's not the *most* expensive thing that could go wrong

Maybe it's coincidental but a few days prior to this issue coming up I did have to start the truck with the weather just below freezing without the block heater. Had to crank it for a solid couple minutes before she fired up. The PCB must not have liked that. Guess I better start looking for a longer extension cord to keep the truck plugged in :/

 

[charlesmann]

There are threads on rebuilding the pcb instead of replacing.

 

[DampLemonade]

i did consider rebuilding but my box is kinda crusty from a mediocre paint job from a few owners back.

That said, if anyone is interested in taking my old one once I get it out of the truck you'd be welcome to it